Understanding the Linux Kernel

18.3. System V IPC

IPC is an abbreviation that stands for Interprocess Communication. It denotes a set of system calls that allows a User Mode process to:

• Synchronize itself with other processes by means of semaphores

• Send messages to other processes or receive messages from them

• Share a memory area with other processes

IPC was introduced in a development Unix variant called "Columbus Unix" and later adopted by AT&T's System III. It is now commonly found in most Unix systems, including Linux.

IPC data structures are created dynamically when a process requests an IPC resource (a semaphore, a message queue, or a shared memory segment). Each IPC resource is persistent: unless explicitly released by a process, it is kept in memory. An IPC resource may be used by any process, including those that do not share the ancestor that created the resource.

Since a process may require several IPC resources of the same type, each new resource is identified by a 32-bit IPC key, which is similar to the file pathname in the system's directory tree. Each IPC resource also has a 32-bit IPC identifier, which is somewhat similar to the file descriptor associated with an open file. IPC identifiers are assigned to IPC resources by the kernel and are unique within the system, while IPC keys can be freely chosen by programmers.

When two or more processes wish to communicate through an IPC resource, they all refer to the IPC identifier of the resource.

18.3.1. Using an IPC Resource

IPC resources are created ...